1. Field of the Invention
The present invention relates generally to a magneto-optical recording medium, and more particularly to a domain wall displacement detection (DWDD) system magneto-optical recording medium in which a magneto-optical recording arrangement film is formed on a substrate having a land and a groove, both of the land and the groove are used as signal recording areas and which uses a reproduction system based upon a DWDD system and a method of manufacturing such DWDD system magneto-optical recording medium.
2. Description of the Related Art
Of magneto-optical recording mediums, in particular, a DWDD (domain wall displacement detection) system magneto-optical recording medium is able to attain high-density recording, and therefore it is regarded as an important magneto-optical recording medium.
A reproduction system of this DWDD system magneto-optical recording medium is based upon a system in which a magnetic recording mark is enlarged in size by a temperature distribution produced with irradiation of reproducing laser light on the magneto-optical recording medium, the thus enlarged recording mark being read out from the magneto-optical recording medium. According to this arrangement, the recording mark is microminiaturized up to the size smaller than an optical diffraction limit of the reproducing light, whereby a linear recording density can be increased to attain a large recording capacity.
In the case of the DWDD system magneto-optical recording medium, in order to improve jitters and bit error rates, it becomes necessary to realize a smooth displacement of a domain wall.
As a method of smoothly displacing a domain wall, there have been proposed methods in which a magnetic layer between the tracks is changed as an in-plane magnetic film or a non-magnetic film by annealing the magnetic layer in order to avoid a magnetic field generated from the adjacent recording track from affecting the smooth displacement of the domain wall (see cited patent references 1 and 2, for example).
In the above-mentioned DWDD system magneto-optical recording medium, in order to increase a recording capacity much more, it is requested that the recording system thereof should be based upon a so-called land and groove recording system in which both of the land and the groove are used as the recording areas. In that case, it is necessary that the domain wall should be displaced smoothly both with respect to the recording areas of the lands and the grooves.
Accordingly, in this land and groove recording system, in order to enable the domain wall to be displaced satisfactorily with respect to the two recording areas, as shown in FIG. 1, a magneto-optical recording arrangement film 102 formed on a recording medium substrate 101 is annealed by irradiating annealing light La on substantially the center, for example, of a boundary wall surface 105 between a land 103 and a groove 104 formed on the substrate 101.
In this case, when the annealing light La moves toward the side of the groove 104 or toward the side of the land 103 as shown by an arrow a in FIG. 1, as shown by a solid line curve and a broken line curve which show relationships between annealing powers in the respective recording areas of the land 103 and the groove 104 and bit error rates obtained after the annealing treatment in contrast in FIG. 2, optimum annealing powers PBL and PBG which show minimum bit error rates in the land 103 and the groove 104 become different from each other. That is, the annealing power margins that can reduce the bit error rate at the same time are narrowed both in the land 103 and the groove 104.
The thus obtained magneto-optical recording medium causes land and groove recording and reproducing characteristic to differ from each other, and hence a problem arises, in which the recording power margins also cannot be increased sufficiently.
It is not always desired that the annealing position should be selected at the center of the boundary wall surface due to various reasons such as characteristics, shapes and dimensions of the magneto-optical recording arrangement film in the land and the groove. In addition, it is frequently observed that the optimum annealing position should be selected at every manufacturing lot, for example.
In order to improve the jitter and the bit error rates equally both in the recording areas of the land and the groove as described above, it becomes necessary that annealing light should irradiate at optimum positions between upper and lower edge portions of the boundary wall surface along the recording track length direction over the whole of the recording tracks.
Then, as a method for irradiating annealing light at the predetermined position, there is considered a tracking servo method based upon a so-called three-beam push-pull detection method that is applied to an optical pickup for use with ordinary optical recording mediums.
However, according to this method, as will become clear from the descriptions which will be made later on, a problem arises, in which sensitivity at which the position of the annealing light on the boundary wall surface should be detected is low.
[Cited Patent Reference 1]
Japanese laid-open patent application No. 8-147777, paragraph number [0010]
[Cited Patent Reference 2]
Japanese laid-open patent application No. 2002-319198, paragraph numbers [0010], [0011]